The basic objective of this proposed research is to elucidate the factors which lead to injury and death of cells infected by Rickettsia rickettsii. Experiments are designed to determine the effects of specific pathogenetic mechanisms in the host-parasite interaction. Using the plaque assay (a quantitative measure of cell injury) and immunofluorescent demonstration of infected foci (suitable for measurement also), we shall determine the importance of rickettsial penetration as a pathogenetic mechanism by inhibition of penetration at the crucial point in time. Likewise, the effects of increase and decrease in important metabolic substrates and of decrease in intracellular ATP will be determined for host cell injury and rickettsial growth. Investigations to discover the effect of rickettsial release from cells and possible toxic products of rickettsiae will be performed by chemical inhibition of release and separation of infected and uninfected cells by a membrane impermeable to rickettsiae, respectively. The basic techniques of rickettsial plaque assay and fluorescent microscopic measurement of area of infected foci will be supplemented by ultrastructural observations of plaques and infected foci as a monolayer. Thus, the cellular aspects of host-parasite relationship may be assessed in a simplified in vitro system which contains the essential elements of the multifocally infected monolayer of endothelium in spotted fever. The pathogenetic mechanisms of this obligate intracellular parasite for cells may reveal principles important to other host-parasite relationships.